I noticed the SC Report mentioned a proportional controller now avalible from Engle. That would give the user pretty precise control of settings, and hover ability, I would think. This made me curious about the piston systems in general.

As best I can tell, the Engle tanks are for dry hull boats. Although a larger lexan wtc could house one. Perhaps a 4 incher? Not sure. I thought here would be a good place to post efforts, and ideas for piston tank set ups, good expriences and bad. Mr Oudmayer, I recall, made one that looks like artwork. RPM had a neat one too. Just a few examples. There are other systems, of course. We all know about them. I would like to see not a debate of one( gas,RCAB,or others) vs the piston style. Just the angles tried in this one area of piston ballasts. Please post.

I have been running my OTW Seehund for 7-8 years using a dive module that uses an Engel piston for the ballast system. It's a 110mm OD Lexan cylinder (roughly 4.25 inches). As my SubComEast mates can attest....it runs quite nicely.

To control the piston I am using an electronic module that counts (and remembers) the number of revolutions of the main gear the drives the jack screw using a Hall-effect sensor. This is what I have named 'Dial-a-Depth' (tm) since I use a rotary knob on my transmitter to command it. As is to be expected, piston settings are exactly repeatable. Once I know the correct setting for that day's run to get be to the correct submerged trim, I can return to exactly that setting at any time without making any physical change to the sub.

Markus Rieger used to do a really neat encoder based piston tank control board which was fully solid state- no relays. I have a feeling they're no longer available.

Engel have sold a proportional boards for years, but up until the unipro, they were always based on a potentiometer for feedback.

Something you do need to be mindful of with a piston tank system, is that the air compresses behind the piston as the tank fills. Therefore it's important to ensure sufficient volume to prevent too high a pressure building up inside the cylinder, which will load the drive motor up and place a burden on the shaft and hull seals.

Generally the tank should be sized at about 10-15% of the cylinders volume if you wish to keep pressure within a couple of psi. For some boats with a high freeboard that can be difficult to achieve, so sometimes a hybrid system employing a second system (e.g. water pump, gas, compressed air etc.), with smaller piston tank for trim is employed. Another method employed by Ron P was a valve system which vented excess pressure as the tanks filled, and then allowed fresh air in when the boat surfaced to break any vacuum that developed inside the boat.

Piston tanks are quite popular here in the UK. People use Engel tanks and Ron Perrott produced piston tanks where the threaded rod stayed fixed, with the piston running up and down- this meant you didn't have to allow room for the rod to retract.

For smaller boats, converted syringes make excellent tanks, or they can also be used as a trim tank for a hybrid system on a larger boat. The neat thing with such a small tank, is that the small size of the motor required means they can be driven using a standard servo amplifier board. With servos available for a pound or two, that's a dirt cheap way of getting a proportional piston tank.

4" (100mm) is the absoulte minimum diameter for an engel tank, they just won't fit into anything smaller, this is largely to do with the way the motor is offset from the tank itself. Most of Engels rnage are dry hull based, but their Type VII most cetainly isn't.

I made a small hollow brass tube attachment that extends out the stern end of the dive module...that's where the jack screw goes when the piston is at the full dive position. The tube is about 2 inches long.

For seals I am using the OTW style rubber bellows.

When I first started building this module, I did some pressure calculations and figured that when the piston was in full dive position the pressure inside the dive module was 1.5 bar. The end caps are held into place using brass rods and knurled knobs....so no concern with blowing out the end caps.

I have been running my OTW Seehund for 7-8 years using a dive module that uses an Engel piston for the ballast system. It's a 110mm OD Lexan cylinder (roughly 4.25 inches). As my SubComEast mates can attest....it runs quite nicely.

To control the piston I am using an electronic module that counts (and remembers) the number of revolutions of the main gear the drives the jack screw using a Hall-effect sensor. This is what I have named 'Dial-a-Depth' (tm) since I use a rotary knob on my transmitter to command it. As is to be expected, piston settings are exactly repeatable. Once I know the correct setting for that day's run to get be to the correct submerged trim, I can return to exactly that setting at any time without making any physical change to the sub.

I will get some photos posted later today.

-Jeff

Dial-a-depth! Wasn't that my idea You will be hearing from my litigator, or was that alligator. BD.

So, let's assume I don't fully understand (ha! like a box of rocks) the Engle and other piston systems,except that they seem to work really well. When a tank emptys, is it only a vacume, or does it actually take on water? Good discussion so far. I would like to see some one-offs of piston designs too. RPM has a cool one. I had it set side for the Akula X tail project.That one used a 3 inch external lexan tube. Somthing like that a little larger would be cool. He had sourced some (what seemed to be, I did not get a chance to try it), a different path for electronics, that looked pretty good. Wish I had a link for the parts and websites he uses. Something like that, or two piston tank set ups that can fine trim a larger, or long hull, like Pete's Ohio would be interesting. So, a 4 inch lexan is minimum diameter for an Engle tank. Any reccomendations on where to get this tubing, and what inside diameter (wall thickness).Anyone have endcaps for something that large? Still would like to see custom/home made jobs too. Thanks.